1. Field of the Invention
The present invention relates to control systems using multiple control loops, and particularly but not exclusively to control loops in an amplification stage for providing a modulated supply voltage.
2. Description of Related Art
Conventional multi-loop or cascaded control systems may be divided according to speed of operation: each loop may operate at a different frequency in accordance with its purpose in the control system. However each loop must typically be able to operate over the full frequency range of the control system, typically down to zero hertz, i.e. each loop must be able to provide a constant output.
In a control system having more than one control loop, typically a first path provides control at low frequencies. This first path may be a feedforward of a feedback control path. A second path typically provides control at a higher frequency, to remove or reduce any error in the first path. This second path is typically a feedback path.
An example application of such a control system is a modulated power supply for providing a supply voltage to an amplification stage, typically a radio frequency (RF) amplification stage. An example of a particularly advantageous modulated power supply stage can be found in United Kingdom Patent No. 2398648.
In this modulated power supply stage there is provided an efficient technique for tracking the supply voltage to an RF amplifier in dependence on the RF input signal to be amplified by the amplifier. A first control loop tracks the envelope of the input signal, representing a desired supply voltage for the amplification stage, and selects one of a plurality of available supply voltages in dependence thereon. A second control loop tracks the envelope of the input signal and the actual output signal, and generates an error signal representing the difference there between. This error signal is combined with the selected supply voltage to provide an adjusted selected supply voltage for the amplification stage. The first control loop is a low frequency loop, and the second control loop is a high frequency loop.
The error signal provided by the second loop contains high and low frequency signals, and has a very large bandwidth. This places a burden on a combiner used for combining the error signal with the selected supply voltage. This combiner must be capable of operating over an extremely high bandwidth, and typically will operate on the edge of its capabilities.
The use of delay stages in control systems is well-known. United Kingdom Patent No. 2398648 discussed above utilises a delay stage in the power supply modulator. European Patent Application Publication No. 1703635 and Japanese Patent Application No. 59152712 also disclose the use of delay stages in control systems.
It is an aim of the invention to provide a technique to reduce the bandwidth and dynamic range burden on the signal combiner in such arrangements.